The present invention relates broadly to eddy current probes and instrumentation, and in particular to an eddy current probe calibration standard.
In the prior art, various methods to detect fatigue cracks in fastener holes and other critical areas of aircraft structural parts have become routine inspection requirements. Depending upon the acceptance limits established for inspection of hole cracks in a wing fitting for example, nondestructive test methods, techniques, and procedures are developed. X-rays are useful when gross cracks can be tolerated, or if accessibility is a problem. Ultrasonics can be used to detect cracks smaller than those found by x-rays, and when fasteners cannot be removed. Penetrants will detect cracks that are open to the surface, but are often times limited in their usefulness due to any foreign material embedded in a cracked area. Optical methods are extremely useful for showing small flaws, but extensive hole preparation is necessary in order to clean foreign materials and smeared metal from the hole surface. The methods for magnetic rubber and magnetic particle inspection of ferromagnetic parts have been developed utilizing soft iron indicators and electronic instruments to measure magnetic fields. While these methods are very desirable for inspection of steel parts, they are time consuming and require extensive surface preparation. Eddy currents inspection methods using both surface, and bolt hole probes, can detect not only small cracks, but requires a minimum of surface preparation. Because of these advantages, eddy currents are often selected as the primary inspection method.
Eddy Current inspection techniques for nondestructive testing of fasteners holes and surface areas such as aircraft wing skins, and other structural parts requires that flaw information be obtained from probe contact to the material under test. Eddy current testing is based on the conductivity of the material which is primarily determined by the material's chemical composition. When a test coil is placed above the surface of an isolated conducting material, the coil's magnetic field induces current into the material. The eddy current field developed by the flow of eddy currents will vary as the flow of eddy currents varies. Cracks inclusions, and changes in conductivity will cause this flow to vary. This is accomplished by a hand held probe, or by using an automatic scanner which rotates the eddy current probe continuously 360.degree. throughout the length of a fastener hole undergoing inspection. This automatic system makes use of the same general principles as mentioned above and has been established by hand-scan methods.
The primary requirement for an eddy current calibration standard is to provide a valid calibration of probe and instrument for a response which can be related to the minimum crack size to be detected. This requirement was not being satisfied prior to the invention since a basic calibration standard had not been established for general use by private industry, the Air Force, or other governmental agencies. Many nondestructive test personnel in the field do not have facilities to manufacture reference blocks such as the present block with a single saw cut edge.
For many years, nondestructive inspection personnel have tried to determine an eddy current probe's sensitivity from a small (45.degree.) forty-five degree jewelers saw cut made at the edge of a hole for a specific size probe. A signal is transmitted from the probe and a crack-like indication is obtained from an eddy current instrument, and a trace made with a high speed recorder. This simulated defect, at best, serves only as a metal flaw reference. However, it could not be related to any specific defect or crack size. Further, this has never provided a valid, repeatable, traceable method of eddy current probe and instrument calibration.